Disc drives of the type known as "Winchester" disc drives, or hard disc drives, are well known in the industry. Such disc drives magnetically record digital data on a plurality of circular, concentric data tracks on the surfaces of one or more rigid discs. The discs are typically mounted for rotation on the hub of a brushless DC spindle motor. In disc drives of the current generation, the spindle motor rotates the discs at speeds of up to 10,000 RPM.
Data are recorded to and retrieved from the discs by an array of vertically aligned read/write head assemblies, or heads, which are controllably moved from track to track by an actuator assembly. The read/write head assemblies typically consist of an electromagnetic transducer carried on an air bearing slider. This slider acts in a cooperative hydrodynamic relationship with a thin layer of air dragged along by the spinning discs to fly the head assembly in a closely spaced relationship to the disc surface. In order to maintain the proper flying relationship between the head assemblies and the discs, the head assemblies are attached to and supported by head suspensions or flexures.
The actuator assembly used to move the heads from track to track has assumed many forms historically, with most disc drives of the current generation incorporating an actuator of the type referred to as a rotary voice coil actuator. A typical rotary voice coil actuator consists of a pivot shaft fixedly attached to the disc drive housing base member closely adjacent the outer diameter of the discs. The pivot shaft is mounted such that its central axis is normal to the plane of rotation of the discs. An actuator bearing housing is mounted to the pivot shaft by an arrangement of precision ball bearing assemblies, and supports a flat coil which is suspended in the magnetic field of an array of permanent magnets, which are fixedly mounted to the disc drive housing base member. On the side of the actuator bearing housing opposite to the coil, the actuator bearing housing also typically includes a plurality of vertically aligned, radially extending actuator head mounting arms, to which the head suspensions mentioned above are mounted. When controlled DC current is applied to the coil, a magnetic field is formed surrounding the coil which interacts with the magnetic field of the permanent magnets to rotate the actuator bearing housing, with the attached head suspensions and head assemblies, in accordance with the well-known Lorentz relationship. As the actuator bearing housing rotates, the heads are moved radially across the data tracks along an arcuate path.
Disc drives of the current generation are included in desk-top computer systems for office and home environments, as well as in laptop computers which, because of their portability, can be used wherever they can be transported. Because of this wide range of operating environments, the computer systems, as well as the disc drives incorporated in them, must be capable of reliable operation over a wide range of ambient temperatures.
Furthermore, laptop computers in particular can be expected to be subjected to large amounts of mechanical shock as they are moved about. It is common in the industry, therefore, that disc drives be specified to operate over ambient temperature ranges of from, for instance, -5.degree. C. to 60.degree. C., and further be specified to be capable of withstanding operating mechanical shocks of 100 G or greater without becoming inoperable. Moreover, future disc drive products are being developed which must be capable of withstanding non-operating shocks of up to 1000 G without suffering fatal damage.
One of the undesirable possible consequences of mechanical shocks applied to a disc drive is the phenomenon commonly referred to in the industry as "head slap". This condition occurs when the applied mechanical shock is large enough to overcome the load force applied to the head assembly by the head suspension. Under such conditions, the head assembly lifts away from the disc surface, and when the shock event terminates, the head assembly moves back into contact with the disc in an uncontrolled manner, potentially causing damage to the head assembly, disc or both.
One common preventive measure used in the industry to prevent head slap is to use ramps closely adjacent the outer diameter of the discs to unload the heads from engagement with the discs when a non-operating condition, such as loss of disc drive power, is detected. Since the heads are no longer resting on the disc surface, applied mechanical shocks cannot cause uncontrolled contact between the heads and discs. Once proper operational conditions are restored, the head assemblies are reloaded into engagement with the discs for normal disc drive operation.
In order to ramp load/unload the head assemblies, the head suspensions which support the head assemblies must include some sort of ramp contact feature to cooperate with the ramps, and these ramp contact features can be divided into two general groups: 1) ramp contact features located adjacent the leading edge of the head assembly, i.e., between the actuator pivot point and the head assembly; and 2) ramp contact features located adjacent the trailing edge of the head assembly, i.e., at the far distal end of the head suspension.
Head suspensions that include ramp contact features from the first group have the advantages of low mass and inertia during actuator seeks, high modal frequencies, good operating shock characteristics and simple access to the bonding pads used for electrical connection of the head transducers. The prior art use of this type of ramp contact feature does, however, have the disadvantages of requiring a parabolic ramp surface to ensure point contact between the ramp surfaces and the ramp contact features and insufficient clearance between the ramp contact feature and the disc surface to allow for assembly tolerances in a multi-disc disc drive assembly. Co-pending U.S. patent application Ser. No. 09/215,428, filed Dec. 18, 1998, assigned to the assignee of the present invention, discloses a novel arrangement of ramp contact features which overcomes these disadvantages, and the present invention will be discussed in detail hereinbelow as it can be utilized with such a head suspension assembly.
Head suspension assemblies that include ramp contact features from the second group have the advantages of allowing for flexibility of design of the contact features to allow for sufficient spacing between the disc surface and the ramp contact features, and the capability of having the ramp contact feature located on the head suspension centerline to limit static attitude biases on the gimbal of the head suspension. However, such assemblies typically introduce unwanted mass and inertia during seek operations, have less desirable modal frequencies, less desirable operating shock characteristics and greatly restricted access to the head assembly electrical bonding pads.
The ramp contact features of the prior art are typically formed as elements of the relatively robust rigid beam portion of the head suspension assembly. When contact is established between the ramp contact features on the rigid beam portion of the head suspension assembly and the ramp surfaces, the lifting force is thus not directly applied to the relatively flexible gimbal portion of the head suspension assembly to which the heads are attached. Therefore, the mass of the heads is solely supported by the gimbal portion of the head suspension assembly when the heads are unloaded from cooperative engagement with the discs.
Prior art heads commonly included slider bodies of a type known as "positive pressure air bearing sliders" (PPABS). Such PPABS heads included air bearing surfaces which interact with a thin layer of air dragged along by the spinning discs to generate a hydrodynamic lifting force which tended to separate the heads from the disc surface. This lifting tendency was counterbalanced by forming a spring portion of the head suspension assembly to generate a balancing "load force" in a direction opposite to the hydrodynamic lifting force. The "flying height" of the heads was thus determined by the relative strengths of the hydrodynamic lifting force and the head suspension load force.
Recent industry demands for increased areal density of the recorded data on discs have, in turn, required that the heads be flown in greater proximity to the disc surfaces, with heads of the current generation utilizing flying heights of 1.0 .mu." (0.000001 inch) or less. With such small flying heights, manufacturing tolerances in the head suspension assemblies lead to increasing difficulties in balancing the hydrodynamic lifting force of the head slider with the load force of the head suspension assembly to the necessary tolerance levels.
The requirement of lowered flying heights with more stringent tolerance needs has lead to the development of a new type of slider body for mounting the transducers used to record and retrieve data on the disc surface. This new type of slider body includes "negative pressure air bearing surfaces" (NPABS). As the name suggests, NPABS sliders include features that not only generate a hydrodynamic lifting force at the air bearing surfaces, but also include specially configured features that generate balancing low pressure, or "negative pressure", areas which tend to draw the head closer to the disc. Proper design of the slider body thus permits implementation of heads in which the hydrodynamic lifting force is balanced against the negative pressure created by the air bearing configuration to create slider bodies which are "self-balancing" and fly with great stability at a desired flying height. Since the balance of upward and downward forces exerted on the head is a function of the more easily controlled dimensions and features of the slider body itself, rather than the slider body and spring portion of the head suspension, variation of flying height from head to head can also be maintained within the more stringent tolerance ranges.
Solving the problem of head flying height through the use of NPABS sliders does, however, create a new engineering challenge related to ramp unloading and loading of the heads.
Since NPABS heads exert a downward (i.e., toward the disc surface) force in operation, a correspondingly greater force must be exerted to lift the heads away from the discs during head unloading, leading to potentially fatal stressing of the delicate gimbal portion of the head suspension if the lifting is accomplished by contact between the ramps and the load beam portions of the head suspension assemblies.
A need clearly exists, therefore, for a head/head suspension assembly which incorporates a NPABS slider assembly--for stability and consistency of flying height--with a ramp contact system which does not exert potentially damaging forces on the delicate gimbal portion of the head suspension during head unload operations.